Plugged in charging but not actually charging!

My Dell Laptop is 3 years old, the battery health status is good, but when its plugged in charging its no longer charging to 100% in fact over the last week or so its gone from a maximum charge of 37% to 7% today!

I had the adapter tested and its working fine, has anyone any ideas as to why this is happening and how It can fix it before I am down to 0% !!

if the lead is out I get the message about your power being critically low!